Bruno Ganz. I cant recall where I heard it, but apparently he watched movies that had been secretly made while Hitler conversed with someone on a train, so he could get his mannerisms down.
It wasn't a movie, it was an audio recording of Hitler in a private conversation with the Finnish Field Marshal Carl Gustaf Mannerheim. Hitler visited him on June 4th 1942, at a railway siding in SE Finland, and some Finnish broadcasting company employee secretly recorded 11 1/2 minutes of conversation between the two. It is the only known recording of Hitler speaking in a non-official tone, as he never allowed audio recordings of him off-guard.
according to the original post: The Downfall was praised for his historical correctness. the film follows the book of Hitlers secretary Traudl Junge "Bis zur letzten Stunde" (engl.: Until the Final Hour: Hitler's Last Secretary) i think Valkyrie is a good Movie, but historical not realy accurate, i think its alike a typical hollywood Movie "based on a true story", so not realy true but the the main facts happened in deed.